1. Field of the Invention
The present invention relates, in general, to the construction of concrete walls, slabs, and other static structures, and, more particularly, to a concrete flooring assembly, and method of fabrication, for use with expansive soils that utilizes concrete support beams to create voids and to support the structural slab.
2. Relevant Background
Commercial and residential buildings are often built on foundations comprising vertical perimeter walls of poured concrete. Since the vertical foundation walls are structural members which support the building, they are usually several feet in depth and function as beams bridging between footers or piers resting on bedrock or stable soil. It is common practice in such buildings to provide a basement, or ground floor, wherein at least a portion of the basement walls include the vertical foundation walls and wherein the basement floor is a poured concrete slab resting on the soil enclosed by the foundation walls. Typically, the foundation is constructed by first excavating a pit for the basement and for the foundation footers. Then, forms are erected around the periphery of the pit and concrete for the foundation walls is poured into the forms.
A major problem with conventional construction in certain soil and climate conditions is that the location of the basement floor can be unstable due to movement of the underlying soil. Expansive soils are prevalent in many areas of the Unites States and other countries. These expansive soils can expand and contract considerably as a result of cyclical changes in moisture content and/or as a result of freezing and thawing cycles. The soil expansion and contraction problem can be especially severe when the floor is simply a slab of concrete poured onto the surface of the soil that forms the floor of the excavation pit. For example, certain dense clay soils tend to dry out after excavation and then later absorb water and swell. This swelling or expansion causes the slab to move relative to the foundation walls which can generate large forces that are sufficient to crack or break the slab. In general, because the foundation walls must support the building, they are supported by piers or pads on solid ground or bedrock or piers or pads on footings and therefore are very stable. However, when the basement floor is a relatively thin slab of concrete having a large surface area and resting on a large area of soil, it is highly vulnerable to movement due to expansion and contraction of the soil as water is absorbed and released by the soil. The relative motion between the slab and the walls can damage interior walls.
A variety of techniques have been implemented to control the effects of expansive soils on concrete foundations and structural slabs or floors. Generally, each of these techniques attempts to separate the foundation walls and structural slabs or flooring from the heaving soils or to at least absorb some of the expansive forces created by the moving soil. Unfortunately, these techniques have proven to be costly, to increase the complexity of fabricating concrete foundations and flooring, to cause long-term structural or safety problems, and to reduce spacing between the floor and ceiling.
For example, a common technique of protecting the foundation and slab from the expanding soil is to create a void space under the concrete slab. To create the void, cardboard forms or other degradable material forms are positioned under the form or pan used during pouring of the foundation walls and floor. With time, the material of the void form begins to deteriorate creating a void in which the soil can expand without moving the wall or floor. However, the degradation of the forms typically is accompanied by mold growth and the release of associated toxins, which can result in safety issues within the structure above the concrete foundation. Additionally, jobsite delays and inclement weather during initial construction can result in premature degradation of the cardboard void form and loss of the strength needed to support the curing concrete wall and floor.
Another technique has involved structurally supported wood floors to replace the concrete slab, but the wood product has tended to degrade in a similar fashion to the cardboard forms when exposed to moisture in the adjacent soil. More recently, raw steel components have been used to create voids spaces, but the expected life and reliability of the steel components is questionable in the moist environment below grade that is likely to cause rapid rusting.
There remains a need for an improved method and system for creating and protecting concrete foundation walls and structural slabs from the effects of expansive soils. Preferably, such a method and system would be relatively inexpensive to implement in the cost-sensitive construction industry and lend itself to the field conditions associated with excavating soil and forming. structures with concrete. Further, the method and system preferably would result in void spaces being created under structural slabs and allow removal of any degradable void forms after formation of the foundation wall and/or slab.